Gas-phase polymerization processes are economical processes for the polymerization of olefins such as homopolymerizing ethylene or propylene or copolymerizing ethylene or propylene with other olefins. One aspect of such a gas-phase polymerization process which needs specific attention is discharging of the produced polymer from the reactor. Various continuously or discontinuously operating methods have been developed for being able to carry out the continuous polymerization processes without severely impacting the polymerization conditions.
EP 071 430 A2 describes a fluidized-bed discharge process in which solid particulate material is transferred from the fluidized-bed reactor to lower pressure through a sequence of two settling vessels. Variants of such a discharge system are, for example, also described in EP 870 539 A2, U.S. Pat. No. 6,498,220 B2 or WO 2007/102942. WO 00/29452 A1 discloses a method of discharging polymer from a continuously operated gas-phase reactor in which polymer powder is continuously withdrawn from the reactor and which is preferably carried out in combination with batch-wise discharging polymer powder. WO 2007/071527 A1 describes a process for discharging polymer from a fluidized-bed reactor by continuously circulating polymer in a circulation loop from the base of the reactor to the upper region of the reactor and continuously withdrawing polymer from this circulation loop. WO 2006/082007 discloses discharging polymer particles from a fluidized-bed reactor via one or more product offtake lines which operate preferably discontinuously. EP 648 697 A1 refers to a method for transporting polyolefin particles from a first container to a second container by means of a pressure difference and a cleaning gas is fed to the transportation pipe for cleaning in. JP 2007/284548 A discloses a flexible transfer piping between a gas-phase polymerization reactor and a hopper which is ascending and does not have a horizontal part.
Requirements for a convenient discharging process are inter alia that the discharging shall work without disturbing the continuous polymerization process, that the discharging process as such shall not negatively influence the properties of the discharged polymer, that the process shall operate reliably over a long period of time and that the amount of discharged polymer can easily be controlled. This includes, for example, that the amount of reaction gas exiting the reactor as by-product should be as low as possible. The process shall further allow varying the amount of discharged polymer per time over a wide range. Moreover, the discharging system shall be insusceptible for deviations in the polymerization process such as for example formation of lumps or a shutdown of the polymerization process.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and to find a process for discharging particles form a gas-phase polymerization process which fulfills the requirements for a convenient discharging process and which is not too costly in its realization.